The invention relates to a measuring instrument for measuring the optical focal spot of an x-ray tube, comprising a pinhole camera for forming an image of the focal spot on a radiation-sensitive detector. Measuring instruments of this type serve the purpose of determining the size of the optical focal spot of an x-ray tube and the intensity distribution on the focal spot.
In the publication "Final draft on focal spot characteristics" of the International Electrotechnical Commission of Oct. 30, 1979, a measuring arrangement of this type is described. A pinhole diaphragm of a pinhole camera is brought into the center of the path of rays being issued by the anode of an x-ray tube and generates a radiograph of the focal spot on an x-ray sensitive fine grain film. Through suitable selection of the spacings between the pinhole diaphragm and the film plane, and between the focus and the pinhole diaphragm, the image of the focal spot can be enlarged. However, on account of the low sensitivity of the x-ray film and the maximally allowable tube load, only an approximately triple enlargement, as the maximum, can be attained. Therefore, for a precise measurement, the radiograph must be yet further enlarged by optical means. Due to the necessary steps, development and exposure of the film, and the optical enlargement for the purpose of masurement, this method is shown to be complicated and time-consuming. Since the film has a logarithmic transmission behavior, the intensity distribution in the focal spot must first be ascertained by way of computation via the characteristic curve of the film. Since the film development has a great influence on the slope of the characteristic curve, a precise check (or inspection) of the development is necessary.